The present invention relates to rubber compounds which contain fillers and which contain solution rubbers which contain carboxyl groups, to the production of the above-mentioned rubber compounds; and to the use thereof for the production of vulcanized rubber materials which are particularly suitable for the production of highly reinforced rubber moldings, most preferably for the production of tires which exhibit particularly high wet slip resistance and abrasion-resistance.
Anionically polymerized solution rubbers which contain double bonds, such as solution polybutadiene and solution styrene/butadiene rubbers, have advantages compared with corresponding emulsion rubbers for the production of tire treads which exhibit low rolling resistance. These advantages comprise, amongst others, the feasibility of controlling the vinyl content and the glass transition temperature which is associated therewith, and of controlling the extent of molecular branching. This results in particular advantages in practical use which are related to the wet grip and rolling resistance of the tire. Thus, U.S. Pat. No. 5,227,425 describes the production of tire treads from a solution of SBR rubber and hydrated silica. Numerous methods of modifying the terminal groups have been developed in order to achieve a further improvement in properties. Examples of methods such as these include those described in EP-A 334 042, which employs modification with dimethylaminopropylacrylamide, or that described in EP-A 447,066, which employs modification with silyl ethers. However, the proportion by weight of terminal groups is small due to the high molecular weight of rubbers, and is, therefore, only capable of exerting a slight influence on the interaction between the filler and the rubber molecule. One object of the present invention, amongst others, was to produce solution rubbers comprising a significantly higher content of groups which are effective for interaction with the filler.
A process for the production of solution polybutadiene rubbers which contain carboxyl groups (3.9 to 8.9% by weight) is described, amongst other features, in DE-OS 2,653,144. However, due to their strength, which is too low, and due to their acceptable stress values, which are likewise too low, these rubbers are not suitable as the major component in tire treads.
The object of the present invention was, therefore, to provide rubber compounds comprising solution rubbers containing carboxyl groups, from which tires can be produced which exhibit improved wet grip and a lower rolling resistance, as well as a high mechanical strength and improved abrasion properties.
The present invention, therefore, relates to rubber compounds which consist of at least one rubber and of 10 to 500 parts by weight, preferably 20 to 200 parts by weight, of a filler with respect to 100 parts by weight rubber, wherein the rubber has been produced by the polymerization of aromatic vinyl monomers with diolefins in solution and by the introduction of carboxyl groups, said rubber has a content of 0.1 to 3% by weight of bound carboxyl groups or salts thereof, a content of aromatic vinyl monomers incorporated by polymerization of 5 to 40% by weight, and a content of diolefins of 60 to 95% by weight, wherein the content of 1,2-bonded diolefins (vinyl content) is 5 to 60% by weight, with respect to the solution rubber used in each case.
Rubber compounds according to the present invention, which are preferred, are those in which the rubber constituent has a content of bound carboxyl groups or salts thereof of 0.2 to 2.5% by weight, a content of aromatic vinyl monomers incorporated by polymerization of 5 to 30% by weight, and a content of diolefins of 70 to 95% by weight, wherein the content of 1,2-bonded diolefins (vinyl content) falls within the range from 5 to 55% by weight.
Examples of aromatic vinyl monomers which can be used for polymerization include styrene, o-, m- and p-methylstyrene, p-tert.-butylstyrene, xcex1-methylstyrene, vinylnaphthalene, divinylbenzene, trivinylbenzene and/or divinylnaphthalene. Styrene is most preferably used.
Examples of diolefins which can be used according to the invention for polymerization include 1,3-butadiene, isoprene, 1,3-pentadiene, 2,3-dimethylbutadiene, 1-phenyl-1,3-butadiene and/or 1,3-hexadiene. 1,3-butadiene and isoprene are most preferably used.
The rubbers which can be used according to the invention in the rubber compounds and which. are based on aromatic vinyl monomers and diolefins with a content of 0.1 to 3% by weight of bound carboxyl groups, have average molecular weights (number average) of 50,000 to 2,000,000, preferably 100,000 to 1,000,000, and glass transition temperatures of xe2x88x9250xc2x0 C. to +20xc2x0 C., preferably xe2x88x9240xc2x0 C. to 0C., and have Mooney viscosities ML 1+4 (100xc2x0 C.) of 10 to 200, preferably 30 to 150.
In addition to carboxyl groups the rubbers of the invention may contain other functional groups like hydroxyl-, carboxylic ester-, carboxamide- or sulfonic acid groups.
Production of the rubbers which are used according to the invention is effected by anionic solution polymerization, i.e., by means of a catalyst based on an alkali metal, e.g., n-butyllithium, in an inert hydrocarbon as the solvent. The known randomizing agents and control agents for the microstructure of the polymer can be used in addition. Anionic solution polymerization methods of this type are known and are described, for example, by I. Franta in Elastomers and Rubber Compounding Materials; Elsevier 1989, pages 73-74, 92-94, and in Houben-Weyl, Methoden der Organische Chemie, Thieme Verlag, Stuttgart, 1987, Volume E 20, pages 114-134.
The carboxyl groups can be introduced into the rubber either by adding chemical compounds which supply carboxyl groups, for example, CO2, to the metal-containing solution, or by treating the finished rubber in a subsequent reaction with chemical compounds which contain carboxyl groups, for example with mercaptans which contain carboxyl groups.
The carboxyl group content can be determined by known methods, e.g. titration of the free carboxylic acid, spectroscopy or elemental analysis and others.
The carboxyl groups are preferably introduced into the rubber after the completion of the solution polymerization of the monomers used, by the reaction of the polymers obtained, optionally in the presence of radical initiators, with carboxylmercaptans of formula
HS-Rxe2x80x2-COOX,
wherein
Rxe2x80x2represents a linear, branched or cyclic C1-C36 alkylene group which may optionally be substituted with up to 3 further carboxyl groups or which can be interrupted by nitrogen, oxygen or sulfur atoms, and
X represents hydrogen or represents a metal or an, optionally substituted with Cl-C36-alkyl-, cycloalkyl- or arylgroups, ammonium ion.
The preferred carboxylmercaptans are thioglycolic acid, 2-mercaptopropionic acid (thiolactic acid), 3-mercaptopropionic acid, 4-mercaptobutyric acid, mercaptoundecanoic acid, mercaptooctadecanoic acid, 2-mercaptosuccinic acid and alkali, alkaline earth or ammonium salts thereof. 2-and 3-mercaptopropionic acid, mercaptobutyric acid and 2-mercaptosuccinic acid, or the lithium, sodium, potassium, magnesium, calcium or ammonium salts thereof, are more preferably used. 3-mercaptopropionic acid or the lithium, sodium, potassium, magnesium, calcium or ammonium, ethylammonium, diethylammonium-, triethylammonium-, octadecylammonium-, and cyclohexylammonium-salts thereof are most preferably used.
In general, the reaction of the carboxylmercaptans with the solution rubbers is conducted in a solvent, for example, in hydrocarbons such as pentane, hexane, cyclohexane, benzene and/or toluene, at temperatures from 40 to 150xc2x0 C., in the presence of radical initiators, e.g., peroxides, especially acylperoxides, such as dilauroyl peroxide and dibenzoylperoxide and ketalperoxides as di-tert.-butyltrimethylcyclohexaneperoxide, azo initiators such as azo-bis-isobutyronitrile, or benzpinacol silyl ethers, or in the presence of photoinitiators and visible or UV light.
The amount of carboxylmercaptans which are used depends on the content of bound carboxyl groups or salts thereof in the solution rubber which is to be used in the rubber compounds.
Carboxylic acid salts can also be produced by the neutralization of the carboxylic acid groups, after the introduction thereof into the rubber.
Suitable fillers for the rubber compounds according to the invention include all known fillers which are used in the rubber industry, comprising both active and inactive fillers.
Examples thereof Include:
microdispersed hydrated silicas, for example those produced by precipitation from solutions of silicates or by the flame hydrolysis of silicon halides with specific surfaces of 5-1000, preferably 20-400 m2/g (BET specific surface) and with primary particle sizes of 10-400 nm. These hydrated silicas can optionally also be present as mixed oxides with other metal oxides such as Al, Mg, Ca, Ba, Zn, Zr or Ti oxides;
synthetic silicates, such as aluminum silicate, or alkaline earth silicates, such as magnesium silicate or calcium silicate, with BET specific surfaces of 20-400 m2/g and primary particle diameters of 10-400 rim;
natural silicates, such as kaolin and other naturally occurring hydrated silicas;
glass fibers and glass fiber products (mat, strand) or glass microspheres;
metal oxides such as zinc oxide, calcium oxide, magnesium oxide or aluminum oxide;
metal carbonates such as magnesium carbonate, calcium carbonate or zinc carbonate;
metal hydroxides, such as aluminum hydroxide or magnesium hydroxide for example;
carbon blacks. The carbon blacks which are used here are produced by the flame black, furnace black or gas black processes and have BET specific surfaces of 20-200 m2/g, e.g., SAF, ISAF, HAF, FEF or GPF carbon blacks;
rubber gels, particularly those based on polybutadiene, butadiene/styrene copolymers, butadiene/acrylonitrile copolymers and polychloroprene.
Microdispersed hydrated silicas and/or carbon blacks are preferably used as fillers.
The aforementioned fillers can be used on their own or in admixture. In one particularly preferred embodiment, the rubber compounds contain, as fillers, a mixture of light fillers, such as microdispersed hydrated silicas, and carbon blacks, wherein the mixture ratio of light fillers to carbon blacks ranges from 0.05 to 20, preferably from 0.1 to 10.
In addition to said solution rubbers which contain carboxyl groups, the rubber compounds according to the invention may also contain other rubbers, such as natural rubber and other synthetic rubbers also.
The preferred synthetic rubbers are described, for example, by W. Hofmann in Kautschuk-technologie, Gentner Verlag, Stuttgart 1980, and by I. Franta in Elasto-mers and Rubber Compounding Materials, Elsevier, Amsterdam 1989. Amongst other materials, they comprise:
BRxe2x80x94polybutadiene
ABRxe2x80x94butadiene/acrylic acid C1-4 alkyl ester copolymers
CR polychloroprene
IRxe2x80x94polyisoprene
SBRxe2x80x94styreneibutadiene copolymers with styrene contents of 1-60, preferably 20-50% by weight
IIRxe2x80x94isobutylene/isoprene copolymers
NBRxe2x80x94butadiene/acrylonitrile copolymers with acrylonitrile contents of 5-60, preferably 10-40% by weight
HNBRxe2x80x94partially hydrogenated or completely hydrogenated NBR rubbers
EPDMxe2x80x94ethylene/propylene/diene copolymers as well as mixtures of these rubbers. Rubbers, which are of particular interest for the production of motor vehicle tires, and which contain surface-modified fillers, include emulsion SBRs and solution SBR rubbers with a glass transition temperature abovexe2x80x9450xc2x0 C., which can optionally be modified with silyl ethers or other functional groups according to EP-A 447,066, polybutadiene rubbers which have a high 1 ,4-cis content ( greater than 90%) and which have been produced using catalysts based on Ni, Co, Ti or Nd, as well as polybutadiene rubbers with a vinyl content of up to 75%, and mixtures thereof.
The rubber compounds according to the present invention may also of course contain other rubber additives, which are employed, for example, for the crosslinking of the vulcanized rubbers produced from the rubber compounds, or which improve the physical properties, for special purposes, of vulcanized rubbers produced from the rubber compounds according to the present invention.
As crosslinking agents are used particularly sulfur or chemical compounds which supply sulfur. In addition, and as mentioned above, the rubber compounds according to the present invention may contain other additives, such as known reaction accelerators, anti-aging agents, heat stabilizers, light stabilizers, ozone stabilizers, processing aids, plasticizers, tackifiers, foaming agents, colorants, pigments, waxes, extenders, organic acids, retarders, metal oxides and activators.
These rubber additives according to the present invention are used in the customary known amounts, wherein the amount used depends on the subsequent purpose of use of the rubber compounds. The amounts of rubber additives usually fall within the range from 2 to 70 parts by weight with respect to 100 parts by weight of rubber, for example.
As mentioned above, additional rubbers apart from the solution rubber which contains carboxyl groups can also be admixed with the rubber compounds according to the invention. The amount of said additional rubbers usually falls within the range from 0.5 to 70, preferably 10 to 50% by weight, with respect to the total amount of rubber in the rubber compound. The amount of rubbers which are added in addition again depends on the respective purpose of use of the rubber compounds according to the present invention.
The use of additional filler activators is particularly advantageous for the rubber compounds according to the invention, which are filled with highly active hydrated silicas. The preferred filler activators are sulphur-containing silyl ethers, particularly bis-(trialkoxysilyl-alkyl) polysulphides such as those described in DE 2,141,159 and DE 2,255,577. Other suitable filler activators include oligomeric and/or polymeric sulphur-containing silyl ethers corresponding to the description in DE 4,435,311 and EP 670,347. Other substances which can be used include mercapatoalkyl-trialk-oxysilanes, particularly mercaptopropyltriethoxysilane and thiocyanatoalkyl silyl ethers (see DE 19,544,469) and aminogroup-containing silylethers, like 3-amino-propyl-triethoxisilane and N-oleyl-N-propyl-trimethoxisilane. The filler activators are used in customary amounts, i.e., in amounts of 0.1 to 15 parts by weight with respect to 100 parts by weight rubber.
The rubber compounds according to the present invention can be produced, for example, by mixing the solution rubbers, which contain carboxyl groups with the corresponding fillers in suitable mixing apparatuses such as kneaders, rollers or extruders.
The rubber compounds according to the present invention are preferably produced by first polymerizing said monomers in solution, by introducing the carboxyl groups into the solution rubber, and after the completion of the polymerization and of the introduction of the carboxyl groups, by mixing the solution rubber, which is present in the corresponding solvent, with the corresponding fillers and optionally with further rubbers and further additives in the corresponding amounts and simultaneously or subsequently removing the solvent with hot water and/or steam at temperatures of 50 to 200xc2x0 C., optionally under vacuum.
The present invention further relates to the use of the rubber compounds according to the invention for the production of vulcanized rubbers, which in turn are employed for the production of highly reinforced rubber moldings, particularly for the production of tires.